Dealias type when type parameters inference occurs

[Alex1005a]

The code below displays the type parameter [Res] =>> TypeAl[F, String, Res] for the toFunctorOps implicit conversion.

import cats.Functor
import cats.syntax.all.*

type Result[F[_], Err, Res] = F[Either[Err, Res]]

def test[F[_]: Functor](x: Result[F, String, Int]) = {
  x.map(identity) // error here
}

To fix this, you can do dealias if the type before dealias does not have the same arity as the constructor with which unification occurs, but the type after dealias does have the same arity. Then the type parameter F will be inferred.

[odersky]

I think @smarter would be the best person to review this. Guillaume, can you take a look? You have thought most about dealiasing and type inference.